This invention relates to coal blends suitable for combustion in slagging-type combustion apparatus. The blends have the advantage in enabling the use of certain otherwise unsuitable coals as part of the feed to such slagging-type combustors. Enabling the use of such coals for this application increases the potential fuel supply for these units, and in many instances may significantly reduce the overall transportation costs of the coal feed thereby resulting in lowered operating costs for the combustors.
One of the major advances in methods of burning coal in this century has been cyclone-furnace firing. Commonly the Cyclone-Furnace is used in the form of a water-cooled horizontal cylinder with crushed coal entering the burner end of the cyclone, and primary and secondary combustion air introduced tangentially to impart a whirling motion to the incoming coal. Gas temperatures exceeding 3000.degree. F. (1650.degree. C.) are developed. Such high temperature melts the ash, and the melt forms into a layer of liquid slag on the walls. Combustion gases leave through the re-entrant throat of the cyclone at the rear carrying only about 20 to 30 percent of the ash as dust; about 70 to 80 percent of the ash is retained as molten slag which drains away from the burner end through a small slag tap opening, into a slag tank where it is solidified for disposal.
Since ash is removed in fluid form and satisfactory combustion depends upon the formation of a liquid slag layer, the viscosity of the slag at furnace temperatures must permit slag flow to the tapping point.
Generally slag will just flow on a horizontal surface at a viscosity of 250 poises. The temperature at which this viscosity occurs is used as a criterion for suitability of the coal. Typically a temperature of 2600.degree. F. (1427.degree. C.) is considered maximum. Coals having an ash viscosity above 250 poise at 2600.degree. F. are rejected as unsuitable for slagging-type combustion apparatus such as cyclone-furnace. Other slagging-type combustion apparatuses include slag tap furnaces and some partial combustion gasifier designs.
Coals are often classified as having either bituminous or lignitic ash. Bituminous-type ash coals usually contain as a principal component iron and its compounds, e.g., metallic iron, ferrous oxide or ferric oxide. The ferric state tends to increase the temperature required for ash softening and fluidity properties while the metallic and ferrous states tend to lower the required temperature. Lignitic-type ash coals generally contain only small amounts of iron and are little affected by the oxidation state of the iron. Many bituminous coals have a critical temperature above 2600.degree. F., i.e., temperature required to obtain a melted ash having at most a viscosity of 250 poise, thereby rendering such coals unsuitable for slagging-type combustion applications.
It has now been found that such coals may be suitable for combusting in slagging-type combustion apparatus when fed as a blend with certain other coals.